Achieving low emissions of oxides of nitrogen (NO.sub.x) without the use of water is the subject of a considerable amount of research and development in the gas turbine industry. NO.sub.x is an ingredient of smog, and environmental regulations are increasingly strict in limiting its emissions worldwide. Several frame type gas turbine manufacturers have begun guaranteeing 25 parts per million (PPM) NO.sub.x or lower, some even as low as 9 PPM while using natural gas, without the use of water. This compares with NO.sub.x levels in excess of 100 PPM that were routine throughout the industry in the 1970s. The gas turbine emission regulations most widely used in the United States now require 25 PPM using natural gas fuel.
It is more difficult to achieve low NO.sub.x in a typical aeroderivative industrial gas turbine as compared to a frame type gas turbine since one is limited to starting with an existing design rather than an entirely new design. NO.sub.x is formed as a result of high temperature in the flame zone of the combustor. The higher pressure ratios in an aeroderivative gas turbine give them an efficiency advantage over frame type gas turbine in simple cycle applications, but the higher operating pressure creates higher air temperatures at the combustor inlet. The cycle tends to produce more NO.sub.x because of the higher flame temperatures. Also the compactness of aeroderivatives makes it more difficult to design hardware to reduce the flame temperature.
In developing dry low NO.sub.x combustors, all manufactures are premixing some combustion air with the fuel, resulting in a cooler flame. The trick is to design the premixing system so that it not only does the job at full power, but also works at part load and can accommodate transients, such as sudden decreases of load, without a engine flameout. In order to have low emissions of other pollutants, such as carbon monoxide, smoke and unburned hydrocarbons, the flame cannot be too cool, and the combustion process cannot leave pockets of incomplete combustion. Considering all the constraints, the development of a successful dry low NO.sub.x combustion system requires considerable effort.
Along with developing a premix nozzle which meets these requirements a support structure is required for the low NO.sub.x fuel nozzle. The support structure must be sufficiently rigid to withstand the vibrations of the high speed turbo machinery, acoustic excitation, aerodynamic pressure loads, and the high temperatures at the compressor outlet (e.g., about 850.degree. F.). In addition, the support structure must also house the fuel lines to the low NO.sub.x nozzle and provide sufficient insulation to the liquid line to prevent coking of the liquid fuel due to the high temperature.